How do I know if my flight is fully booked?

The easiest way to figure out how full the flight will be is to look at the map Pick up the phone and call the airline. Explain that you're concerned about being on a crowded flight and ask the agent to give you an estimate as to how full the flight currently is.

What happens when the plane is full?

When the aircraft is over booked, the airline usually asks if passengers will give up their seats. They receive a benefit or compensation. The airline will refuse service to passengers that cannot be seated if there is not enough volunteers.